Friction reduction means

ABSTRACT

A component which is adapted for use in a down hole environment such as in a well bore (WB), the component being provided with friction reduction means ( 20 ) comprising a rotatable member ( 28 ) mounted in a ball bearing race ( 27 ), the ball contacting the wall of the well bore (WB) such that the rotatable member ( 28 ) rotates upon the ball bearings to allow the component to be repositioned as necessary in any direction.

[0001] This invention relates to a friction reduction means and, more specifically to a friction reduction means which finds particular application in the field of downhole components that are used in the completion of oil and gas wells. The invention is most particularly relevant to downhole equipment such as centralisers wherein one or more strings of casing are “cemented” within the well bore and the strings have to be centralised and stabilized within the bore.

[0002] In the completion of such wells, a cement slurry is pumped downwardly within each casing string and upwardly within the annulus thereabout, and thus between the well bore and the outermost string or between and inner string and the next outer string. Desirably, the cement column is of substantially uniform thickness about the casing, and, in any event, the casing should not lay up against the side of the well bore or outer casing. It is important that the casing be cemented in the centre of the well bore in order to make better producing wells, prevent interzonal and intrazonal communication, and provide uniform loading which may prevent collapsed casing in the wells and zones that have a history of collapsing casing before the end life of the well. Properly centred casing is important not only in and across the production zones, but also in other zones to reduce bad cement jobs around protective casing shoe joints, and thus reduce expensive cement squeeze jobs for production and for getting good casing shoe seats.

[0003] It is therefore common practice in the industry to dispose so called centralisers along the desired length of the casing. The centralisers are so placed on the casing that after the casing is run in the well bore, the centralisers will be in a position to support the casing off the side of the well bore in zones of interest.

[0004] A well bore is not a true vertical hole in the earth, but has areas of directional turns, bends and vertical deviation changes. When casing is run into the well bore it has to contour with the well bore. The outline of the casing in the well bore may result in severe or continuous contact with the well bore, with more severe contour changes resulting in more severe contacts. This realisation makes a supportive casing centraliser a necessary tool towards centering the casing in a well bore to attain the desirable cement coverage around the casing.

[0005] Known centralisers comprise an annular body which surrounds the casing string, the body having a plurality of blades projecting from the outer surface thereof. The blades contact the inner surface of the well bore and stabilize the casing string in the well bore whilst providing a path for drilling fluids to flow past the centraliser.

[0006] Although known centralisers are suitable for use during operations running casing down vertical sections of the well bore, it will be understood that problems arise when the well bore deviates from vertical. In these cases, the centraliser is not aided by gravity to pass along the well bore. By definition the centralisers contact the inner surface of the well bore and therefore frictional forces are encountered when running casing in such circumstances. Additional force must be used in order to feed the well casing through the well bore in such areas. This can lead to high production times and reduced life of the centralisers.

[0007] With this in mind, friction reducing means have been proposed for incorporation into centralisers and are shown in WO/95/21986. Whilst this document goes some way to overcoming frictional problems associated with casing centralisers, the solution which is proposed is not ideal for use in some sections of well bore for example, where the bore deviates greatly from vertical. The friction reducing means is in the form of a plurality of rollers, each mounted to the centraliser through an axle which must be located in suitable apertures in the centraliser blade. Whilst this allows for friction reduction in a vertical direction, the rollers can only move in one direction and therefore no friction reduction is experienced during rotation of the centraliser for repositioning. Also, any point loading on the roller tends to bend the axle and therefore render the friction reduction element redundant.

[0008] Furthermore, the downhole environment is one which is, by its very nature, dirty and small particles of oil, dirt, soil etc can become lodged within the rollers and prevent the rollers from operating properly.

[0009] It is an object of the present invention to provide a friction reducing means which, when mounted in a downhole component such as a centraliser which, like those described above may be disposed about a casing joint or indeed connected into the casing string through a threaded coupling, which overcomes or at least mitigates the above described problems.

[0010] According to one aspect of the present invention there is provided a component which is adapted for use in a down hole environment such as in a well bore, the component being provided with friction reduction means comprising an outer ball mounted in a ball-bearing race, the outer ball contacting the wall of the well bore. The outer ball allows the component to move smoothly down the well bore and allows the component to be repositioned as often as necessary either rotationally, horizontally or vertically as the outer ball and ball bearing race allow for universal movement as opposed to movement in one direction which was possible with the known roller based friction reduction systems.

[0011] The ball bearing race comprises a plurality of micro balls upon which the outer ball rotates.

[0012] According to a further aspect of the present invention there is provided a casing centraliser comprising a body having an inner surface and an outer surface, at least one blade provided on the outer surface of the centraliser for stabilizing the centraliser within a well bore and providing a path for the flow of drilling fluids, friction reducing means being provided within the blade, the friction reducing means comprising an outer ball mounted in a ball-bearing race, the outer ball contacting the wall of the well bore.

[0013] Preferably, a plurality of friction reducing means are provided on the blade.

[0014] Advantageously, the centraliser is provided with a plurality of blades extending from one end of the centraliser body to the other.

[0015] One embodiment of the present invention will now be described with reference to the accompanying drawings in which:

[0016]FIG. 1 is a perspective schematic view of a known centraliser;

[0017]FIG. 2 is a cross-sectional view of a friction reducing means of a centraliser according to one aspect of the present invention;

[0018]FIG. 2a is a cross-sectional view of the friction reducing means of Figure s, with an alternative fixing means;

[0019]FIG. 3 is a part cross-section view of a centraliser comprising the friction reducing means of FIG. 2;

[0020]FIG. 4 is a side view of the centraliser of FIG. 2; and

[0021]FIG. 5 is a side view of the centraliser of FIG. 2, with an alternative form of blade construction.

[0022] Turning now to the Figures, in FIG. 1, a known centraliser is shown. The well bore WB is substantially vertical, although it will be understood that it may deviate a rather substantial amount with respect to the vertical. A casing or liner C is cemented CM into the well bore. As can be see, the diameter of the well bore is uneven throughout its length, with restricted areas at random points along its length. The casing string CS is made up of a plurality of casing joints CJ, each having a box at one end and a pin at the other end for connection to adjacent joints making up the string.

[0023] Each of the centralisers is indicated in its entirety by reference number 10 and is disposed about a casing joint for maintaining the joint substantially centred within the well bore so that a cement column of substantially even thickness may form about the casing string. It will be understood that the number and spacing of the centralisers along the length of the casing string may vary at the will of the operator of the well.

[0024] In any event, such centralisers comprise a tubular body 11 which is constructed to fit closely about the casing joint, and a plurality of blades 12 which extend longitudinally along the outer diameter of the body thereto in generally equal spaced apart relation.

[0025] More particularly, in the illustrated embodiment the body is of circumferentially continuous construction and of sufficient thickness for adequate strength. The side edges of adjacent blades are disposed apart to allow ample fluid passage, and yet provide bearing surfaces of ample width at the wall of the well bore. In order to assemble the centraliser on the casing joint CJ, the body of the centraliser is slipped over the pin end of the joint prior to make up of the pin with a box end on an adjacent casing joint.

[0026]FIG. 2 is a cross-section through a friction reducing means 20 adapted to be mounted into the blade 12 of a centraliser 10 according to one aspect of the present invention. A straight blade centraliser is illustrated although the present invention is equally suitable for use with spiral blade centralisers. Each blade 12 may be provided with friction reducing means 20 or they may be provided on selected blades only.

[0027] The friction reducing means 20 comprises a seat 21 formed of solid steel or alternatively case hardened steel, the seat being sized to be mounted within the blade 12 of the centraliser. The seat has an upper surface 22 which is semi-circular in configuration as shown in FIG. 2. The thickness of the seat is significant in allowing greater loads to be transmitted through the element into the seat as will be described later. This also increases the wear resistance of the seat which is important in the environment it is adapted to operate in.

[0028] The outer surface 23 of the seat is substantially tubular in configuration. The lower region 24 of the seat is provided with an integral flange 25 which extends outwardly from the outer wall of the seat to form an upper facing shoulder 26.

[0029] A layer of grease (not shown) is provided over the upper surface 22 of the seat and a plurality of micro ball bearings 27 are provided within the seat 21 to cover the upper surface 22 thereof. There may be around 100 micro balls provided to cover the surface of the seat. A rotatable member such as an outer ball 28 is then rotatably mounted within the seat, upon the micro ball bearings 27. The outer ball rotates smoothly upon the micro ball bearings and, in the example illustrated, is significantly larger in diameter than the micro ball bearings. In this example the outer ball is around 15 mm in diameter as compared to the micro ball bearings which are around 2 mm in diameter each.

[0030] A cover member 29 is mounted over the outer ball 28, the cover member is a substantially hollow tubular member. The lower end 30 of the outer wall 31 of the cover member is chamfered inwards 32 so that when the cover member is mounted on the seat, the chamfered edge lies at or adjacent to the shoulder 26 of the seat.

[0031] The lower portion 33 of the inner surface 34 of the cover member lies against the tubular section of the seat 21 and is provided with an internal facing chamfer 35 which fits over the upper surface of the seat at the outer edges thereof. An arcuate recess 36 extends from the internal facing shoulder over the micro ball bearings 27 mounted in the seat and functions to allow the ball bearings to rotate around the seat in use.

[0032] The upper portion 37 of the inner surface 34 of the cover member is adapted to provide an arcuate surface 38 which supports and retains the outer ball 28 of the friction reducing means, without restricting the rotation of the outer ball. The upper portion of the cover has a chamfered surface 39 which improves radial clearances on the blades. During assembly, the cover member 29 may be press fit over the seat portion 21 of the friction reducing means.

[0033] As described above, the friction reducing element is adapted to be mounted within the blade 12 of a centraliser 10. As shown in FIG. 3 a blind bore 30 is provided in each of the blades and a friction reducing element is mounted into each bore 30. The friction reducing means may be secured in a push-fit manner within the bore.

[0034] Alternatively, the external surface of the cover may be threaded 40 (FIG. 2a) with corresponding threads provided within the bore to allow the element to be threaded into position.

[0035] In FIG. 5 an alternative form of blade construction is illustrated. In this example, the blades have a tapered nose section 41 which improves the penetration of the centraliser into the casing and also cigars a path for the friction reduction unit.

[0036] In use, the ball outer 28 of the friction reducing means projects significantly beyond the cover member 29. This allows the surface of the ball to contact the internal wall of the well bore casing without blocking the path for well fluids which is provided between the various blades 12 of the centraliser 10.

[0037] The micro ball bearings 27 upon which the outer ball is mounted ensures that the friction reducing element efficiently reduces frictional forces which act upon the surface of the blades of the centraliser which come into contact with the inner surface of the well bore casing. This enables the casing string to be quickly and efficiently run in the well bore.

[0038] The above described centraliser is subject to lower frictional forces when being run in the well bore which is particularly important when running casing or liner in extended reach and long horizontal sections of the well as centraliser is subject to reduced drag and torque forces and the wear on the centraliser is minimised in comparison to known centralisers.

[0039] The vertical load acting on the large diameter ball is transmitted through the micro balls (ball bearings) to the seat and does not transmit to the cover member.

[0040] Tests have indicated that an element as described in a centraliser can withstand compression loads up to at least 18,000 KG which is substantially greater than that of any known friction reduction unit.

[0041] A further advantage of such centralisers is that the use of water based drilling muds is permitted during drilling operations and this is both economically and environmentally superior to the use of oil based muds.

[0042] The production costs for this element is less than for the corresponding roller arrangement as there is a general reduction in the number of components used, the design is simplified and this leads to a reduction in the time taken to fit the friction reduction elements into the component.

[0043] The above description has been directed towards a casing centraliser which is mounted around a casing string but could also be directed towards any component which is adapted to be used in a similar environment, such as for example an in-line centraliser which is connected into the pipe line via suitable threaded couplings, pipe collar, float equipment and stabilisers. 

1. A component which is adapted for use in a down hole environment such as in a well bore, the component being provided with friction reduction means comprising a rotatable member mounted in a ball bearing race, the ball contacting the wall of the well bore such that the rotatable member rotates upon the ball bearings to allow the component to be repositioned as necessary in any direction.
 2. A component according to claim 1, wherein the ball bearing race comprises a plurality of micro balls upon which the ball rotates.
 3. A component according to claim 1 or 2, wherein the friction reduction means further comprises a seat into which the ball bearing race is mounted.
 4. A component according to claim 3, wherein the seat is formed of solid steel or case hardened steel.
 5. A component according to any one of claims 3 to 5, wherein the component comprises a cover member which cooperates with the seal to retain the rotatable member in contact with the ball bearing race.
 6. A component according to claim 5, wherein the cover member is provided with a central aperture through which the rotatable member projects.
 7. A component according to any one of the preceding claims wherein the diameter of the rotatable member is significantly larger than the diameter of the ball bearings.
 8. A component according to claim 7, wherein the rotatable member is about 15 mm in diameter and the ball bearings are about 2 mm in diameter.
 9. A component substantially as herein before described with reference to and as shown in the FIGS. 2 and 2a of the Figures.
 10. A casing centraliser comprising a body having an inner surface and an outer surface, at least one blade provided on the outer surface of the centraliser for stabilizing the centraliser within a well bore and providing a path for the flow of drilling fluids, said centraliser including a component according to any one of the preceding claims.
 11. A casing centraliser according to claim 10, wherein a plurality of friction reducing means are provided on the blade.
 12. A casing centraliser according to claim 10 or 11, wherein the centraliser is provided with a plurality of blades extending from one end of the centraliser body to the other.
 13. A centraliser substantially as hereinbefore described with reference to and as shown in FIGS. 3 to 5 of the Figures. 